1. Field of the Invention
The present invention relates to a client/server type system, in which economical scalability and system availability are possible.
2. Related Background Art
With advances in computers and networks, there has been a growth in the application range of client/server type systems, in which clients send messages to a server and the server responds to the clients with processing results as messages, and improving the availability of the systems has become increasingly important. At the same time, an economical scalability that can increase or decrease the processing capability of a server according to request volumes from clients is also sought to minimize system costs.
Among methods to improve the availability of client/server type systems, for example, a master/slave method is known. According to the master/slave method, a client is connected to a plurality of servers (for example, a and b) via a communications network, such as an IP network, for example. Each of the servers a and b shares information required to respond to requests from the client and has a synchronization section for detecting occurrences of failures in each other.
According to this method, the server a that responds to requests from the client and the server b as a backup are provided; in the event of a failure in the server a, the server b detects the failure and responds to the client in place of the server a. The client and the servers a and b retain their respective messages until the transmission destinations definitely receive the messages. Through this, according to this method, as long as both the servers a and b do not fail simultaneously, one of the servers a and b can respond to requests from the client.
Due to the fact that the server b is always necessary as a backup for the server a according to the above method, the method cannot be said to have economical scalability, although the number of backup servers may be reduced by an appropriate technology.
Another method for improving availability is a cluster method. The cluster method may be used with a system that comprises, for example, a client, servers a and b and load distributing apparatuses a and b, all connected to each other via an IP network. The servers a and b according to this method do not have synchronization sections. Each of the load distributing apparatuses a and b has a function to detect occurrences of failures in each other and to continue processing in the place of the other in the event of a failure, a function to detect occurrences of failures in the servers a and b, a function to relay messages from the client to one of the servers without any failure, and a function to relay messages from one of the servers to the client. In the event a failure occurs in the server a according to the above method, either the load distributing apparatus a or b relays subsequent messages from the client to the server b, in which failure has not occurred. Furthermore, the client and the servers a and b retain messages until the transmission destinations definitely receive the messages, and resend their respective messages in the event of failed reception. Although there are only two servers, the servers a and b, according to this example, there may be three or more. As a result, according to this method, as long as both the servers a and b do not fail simultaneously, one of the servers a and b can respond to requests from the clients.
The above method can increase or decrease the processing capability of the system by increasing or decreasing the number of servers, and therefore has economical scalability.
The cluster method has economical scalability and has enough availability, even when a failure occurs in a server, to respond to requests from clients subsequent to an occurrence of failure.
However, the above methods lack availability in that requests from clients whose reception was completed by a server before the server failed cannot be responded to, due to the fact that such request messages are lost as a result of the server's failure.